1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display device of a type utilizing a liquid crystal layer made of a mixture of liquid crystal of a high molecular weight with liquid crystal of a low molecular weight and having a so-called memory effect.
2. Description of the Prior Art
In a conventional liquid crystal display device of a type utilizing a twisted nematic (TN) liquid crystal material or a super twisted nematic (STN) liquid crystal material, a plurality of scanning electrodes and a corresponding number of signal electrodes are formed on respective surface of paired substrates so as to extend perpendicular to each other with a layer of liquid crystal material sandwiched between the substrates. To drive this liquid crystal display device, a voltage averaging method is largely employed in which a voltage is applied to the scanning electrodes on a time-sharing basis so that a voltage can be selectively applied to the liquid crystal material forming a pixel at a point of intersection between the scanning and signal electrodes.
It has, however, been found that, in the practice of this driving method, if the number of the scanning electrodes to which the voltage is applied on the time-sharing basis is increased, the length of time required to select one scanning electrode tends to be increased with the consequence that the length of time during which no scanning electrode is selected increases. As a result thereof, the difference in average voltage applied for each frame between an ON pixel and an OFF pixel decreases to such an extent that a display contrast may decreases.
On the other hand, if a liquid crystal material having a memory effect by which, even though the voltage applied is removed, the state (either a light transmitting state or a light scattering state) once assumed during the application of the voltage can be retained is used, the above discussed problem may be eliminated. In other words, if a simple matrix-type liquid crystal display device is constructed with the use of the liquid crystal material having the memory effect, each of the pixels can retain the previously assumed state even though no voltage is applied thereto, and therefore, any reduction in display duty which may occur when the number of the scanning electrodes is increased will not result in a reduction in display contrast, making it possible to provide the liquid crystal display device having a large display capacity. That is to say, since each pixel can be assuredly controlled as to its state (either ON state or OFF state) even during a period in which no voltage is applied thereto, it is possible to increase the display capacity without inviting any reduction in display contrast.
The simple matrix-type liquid crystal display device utilizing the liquid crystal material having the memory effect is disclosed in, for example, the Japanese Laid-open Patent Publication No. 61-103124 and by W. A. Crossland and S. Canter in '85 Society for Information Display International Symposium Digest of Technical Papers, pp. 124-127, (1985), Session: 8.2. According to those literature, a ferroelectric liquid crystal material or smectic dynamic scattering liquid crystal material is utilized and, by the utilization of the memory effect possessed by this specific liquid crystal material, a display is effected by means of a simple matrix driving.
However, in the above described liquid crystal display device, the manufacture is complicated and difficult because, in the case where the ferroelectric liquid crystal material is employed, an adjustment of cell gaps to the submicron order is required and because, in the case where the smectic dynamic scattering liquid crystal material is employed, an application of the voltage in excess of 100 volts is required. Therefore, the liquid crystal display device utilizing any one of the ferroelectric liquid crystal material and the smectic dynamic scattering liquid crystal material has not yet been placed on the market.
Apart from the foregoing, a liquid crystal display device wherein a liquid crystal layer is made of a mixture of liquid crystal of a high molecular weight with liquid crystal of a low molecular weight to accomplish the above described memory effect has recently been suggested, for example, by T. Kajiyama et al. in Chemistry Letters, pp. 817-820, 1989, and also in the Japanese Laid-open Patent Publications No. 2-127494 and No. 2-193115. The liquid crystal layer so suggested is of a type which, when a relatively high voltage of a high frequency (for example, 1 kHz) is applied, it assumes a light transmitting state in which the incident rays of light can pass therethrough, but when a relatively high voltage of a low frequency (for example, 1 Hz to a direct current) is applied, it can assume a light scattering state in which the incident rays of light can be scattered. Also, under a condition in which a relatively low voltage is applied, the previously assumed state (either the light transmitting state or the light scattering state) can be retained.
Thus, where the simple matrix-type liquid crystal display device is constructed using the liquid crystal layer made of the above described mixture, one of the light transmitting state, the light scattering state and the memory state can be selected and, therefore, the liquid crystal display device having a large display capacity and capable of exhibiting a high display contrast such as described hereinbefore can readily be realized.
However, when the use is made of the above described mixture for the liquid crystal layer in making the simple matrix-type liquid crystal device and the memory effect of the liquid crystal layer is to be utilized, it is necessary to bring each pixel into one of the light transmitting state and the light scattering state and also to bring a pixel corresponding to some of the scanning electrodes, which are not selected, into the memory state. This in turn necessitates a switching of one of two types of frequencies and also to employ different voltages to be applied to the pixels corresponding to the selected scanning electrodes and to be applied to the pixels corresponding to the non-selected scanning electrodes, respectively.
A simple matrix driving method in which signals having two frequencies are employed is disclosed, for example, by M. Nagata and N. Nakamura in Mol. Cryst. Liq. Cryst, Vol. 139 (1986), pp 143. However, this method is not a method for controlling the pixels into one of the ON, OFF and memory states and causes the pixels corresponding to the non-selected scanning electrodes to be brought into one of the ON and OFF states. Therefore, the memory effect possessed by the liquid crystal material such as described above is not utilized.
Thus, in the liquid crystal display device having the liquid crystal layer made of the above described mixture, a driving method effective to maximize the utilization of the memory effect of the liquid crystal layer has not hitherto been known and, therefore, it has been recognized difficult to realize the matrix-type liquid crystal display device wherein the above described mixture is used in the liquid crystal layer.